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What do you feed?
It would be hard to know what's wrong unless you lose him and have additional testing/necropsy.
The symptoms you describe do sound neurological so Marek's disease or trauma are possibilities.
I would try vitamin therapy to see if that makes a difference. Give him 400IU Vitamin E and 1/4 tablet B-Complex once a day along with some scrambled egg. Ensure that he's drinking well and eating a nutritionally balanced feed. For a little rooster like that I would give him chick starter to eat.